The present invention pertains to organic electroluminescent devices, such as discrete light emitting devices, arrays, displays, and in particular to the encapsulation of these devices. It furthermore relates to a method for encapsulating the same.
Organic electroluminescence (EL) has been studied extensively because of its possible applications in discrete light emitting devices, arrays and displays. Organic materials investigated so far can potentially replace conventional inorganic materials in many applications and enable wholly new applications. The ease of fabrication and extremely high degrees of freedom in organic EL device synthesis promises even more efficient and durable materials in the near future which can capitalize on further improvements in device architecture.
Organic EL light emitting devices (OLEDs) function much like inorganic LEDs. Depending on the actual design, light is either extracted through a transparent electrode deposited on a transparent glass substrate, or through a transparent top electrode. The first OLEDs were very simple in that they comprised only a two to three layers. Recent development led to organic light emitting devices having many different layers (known as multilayer devices) each of which being optimized for a specific task.
With such multilayer device architectures now employed, a performance limitation of OLEDs is the reliability. It has been demonstrated that some of the organic materials are very sensitive to contamination, oxidation and humidity. Furthermore, most of the metals used as contact electrodes for OLEDs are susceptible to corrosion in air or other oxygen containing environments. A Ca cathode, for example, survives intact only a short time in air, leading to rapid device degradation. It is also likely that such highly reactive metals undergo a chemical reaction with the nearby organic materials which also could have negative effects on device performance. A low work function cathode metal approach requires careful handling of the device to avoid contamination of the cathode metal, and immediate, high quality encapsulation of the device if operation in a normal atmosphere is desired. Even well encapsulated low work function metal contacts are subject to degradation resulting from naturally evolved gases, impurities, solvents from the organic LED materials.
Many approaches have been attempted in order to solve the problem of electrode instability and degradation. A common approach is the use of a low work function metal subsequently buried under a thicker metal coating. In this case, pinholes in the metal still provide ample pathways for oxygen and water to reach the reactive metal below, as is described in Y. Sato et al., xe2x80x9cStability of organic electroluminescent diodesxe2x80x9d, Molecular Crystals and Liquid Crystals, Vol. 253, 1994, pp. 143-150, for example.
The overall lifetime of current organic light emitting devices is limited. The lack of inert, stable, and transparent encapsulants for stable OLED operation remains a major obstacle to OLED development. Organic LEDs have great potential to outperform conventional inorganic LEDs in many applications. One important advantage of OLEDs and devices based thereon is the price since they can be deposited on large, inexpensive glass substrates, or a wide range of other inexpensive transparent, semitransparent or even opaque crystalline or non-crystalline substrates at low temperature, rather than on expensive crystalline substrates of limited area at comparatively higher growth temperatures (as is the case for inorganic LEDs). The substrates may even be flexible enabling pliant OLEDs and new types of displays. To date, the performance of OLEDs and devices based thereon is inferior to inorganic ones for several reasons:
1. High operating current: Organic devices require more current to transport the required charge to the active region (emission layer) which in turn lowers the power efficiency of such devices.
2. Reliability: Organic LEDs degrade in air and during operation. Several problems are known to contribute.
A) Efficient low field electron injection requires low work function cathode metals like Mg, Ca, Li etc. which are all highly reactive in oxygen and water. Ambient gases and impurities coming out of the organic materials degrade the contacts.
B) Conventional AgMg and ITO contacts still have a significant barrier to carrier injection in preferred ETL and HTL materials, respectively. Therefore, a high electric field is needed to produce significant injection current.
3. Poor chemical stability: Organic materials commonly used in OLEDs are vulnerable to degradation caused by the ambient atmosphere, diffusion of contact electrode material, interdiffusion of organics, and reactions of organics with electrode materials.
As can be seen from the above description there is a need for simple and efficient encapsulation of organic light emitting devices. It is a further problem of light emitting devices in general, that a light path for emission of the light generated is to be provided.
It is an object of the present invention to provide a simple and cheap encapsulation of organic light emitting devices.
It is a further object of the present invention to provide new and improved organic EL devices, arrays and displays based thereon with improved stability and reliability.
It is a further object to provide a method for making the present new and improved organic EL devices, arrays and displays.
The invention as claimed is intended to improve the reliability of known organic light emitting devices. The above objects have been accomplished by providing a transparent Siloxane or Siloxane derivative encapsulation for an organic light emitting device. The encapsulant comprises an optical element being arranged such that it lies within the light path of the light emitted by said organic light emitting device. Examples of optical elements that may be formed in, or embedded by the encapsulant are: lenses, filters, color converters, gratings, prisms and the like.
The present invention builds on the finding that Siloxanes and Siloxane derivatives are well suited for use in direct contact with the organic materials used for making organic light emitting devices. This is in contrast to currently accepted OLED technology, where no material is allowed to come into direct contact with the organic device. Current OLEDs are protected by xe2x80x98mechanicalxe2x80x99 sealing, e.g. using an appropriate housing and sealing means.
In contrast to conventional approaches, the encapsulant is also allowed to cover the light emitting portion(s), or part thereof. It turned out that Siloxanes and Siloxane derivatives do not seem to have a detrimental impact on the behavior and lifetime of the light emitting portion of organic devices.
The Siloxanes and Siloxane derivatives form a transparent and non-reactive seal which makes conformal contact with the organic devices. It provides for an excellent barrier to external contamination, such as water, solvent, dust and the like. The proposed encapsulant also protects against corrosion of the highly reactive metal electrodes (e.g. calcium, magnesium, lithium) used in OLED devices. It is non-conductive, which is of particular importance in case that metal electrodes are also embedded in the encapsulant.
Furthermore, Siloxane and Siloxane derivatives are extremely robust and stable. They are unlikely to react with the organic devices even in high-driving, high-heating conditions. Even close to the light emitting portion(s) of OLEDs, where usually the power density has its maximum, no reaction with the present encapsulant takes place.
It is another important feature of Siloxane and Siloxane derivatives that it forms a conformal contact with the underlying organic material such that no air, solvent, or water is trapped. Due to this, the lifetime of the organic device is extended.
Further advantages of the Siloxane and Siloxane derivative encapsulation will be addressed in connection with the embodiments of the present invention.